Integrated optical components are designed to encompass in particular passive waveguides and active waveguides. In such a component, an active waveguide and a passive waveguide are integrated on a substrate using an end-to-end coupling or "butt coupling" method.
The butt coupling method is in common use for coupling a passive waveguide to an active waveguide. The diagrams in FIGS. 1A to 1C show longitudinal-section views of an integrated optical component during the various steps of the butt coupling method.
That method consists initially in growing a first layer 2 formed of a quaternary material on a substrate 1 to act as a passive waveguide, and in burying it in a second layer 3 formed of InP.
Local etching 4 of the two layers 2 and 3 is then performed using a conventional etching method in a zone reserved for integrating another waveguide that is of the active type.
By resuming epitaxial growth, it is possible to form the active waveguide. For that purpose, a layer 5 constituting the active waveguide and formed, for example, of a quaternary material is deposited on the substrate 1, in the locally-etched zone 4, and it is then buried in a cladding layer 6 formed, for example, of InP. The structure of the active waveguide 5 is different from the structure of the passive waveguide 2. The coupling interface 7 between the two types of waveguide is referred to as a "butt joint".
That manufacturing method is currently well mastered when the interface is shallow, i.e. when the thicknesses of the two waveguides are of the same order of magnitude, and, in any event, when the thickness of the thicker waveguide does not exceed 1 .mu.m.
However, when the interface is deep, i.e. when the local etching preceding resumption of epitaxial growth is performed over a thickness greater than or equal to 1 .mu.m, difficulties occur, and that manufacturing method becomes difficult to master. For example, such deep-interface structures are formed for integrating a polarization-insensitive waveguide and a semiconductor optical amplifier in a wavelength converter of the Mach-Zehnder type or of the Michelson type.
In an optical component having a deep interface between the passive waveguide and the active waveguide, and manufactured using prior art methods, the propagation modes have the same dimensions in both types of waveguide. Therefore, in theory, the components have no optical losses. That theory is not borne out in practice because large losses appear at the interface 7. Those losses are due to the fact that, after deep etching, epitaxial growth is very difficult to resume without degrading the crystal quality of the material to be deposited. When the crystal quality of the material is affected, light propagation losses appear at the interface between the two types of waveguide, which losses are greater than one dB and can reach much higher values (a few tens of dB).
In addition, since epitaxial growth is not resumed under good conditions, it is very difficult to align the passive waveguide and the active waveguide correctly, the two waveguides having very different thicknesses.